ruby on rails - `require': cannot load such file -- cancan (LoadError) -

i'm using cancan gem activeadmin. working expected in dev, when pushed staging server following error:

`require': cannot load such file -- cancan (loaderror) 

in case, caused the 'require' line in activeadmin's cancan adapter.

i've searched google potential explanations have come empty-handed.

what might cause of such error, , how can go fixing it?

update:

here config/initializers/active_admin.rb:

activeadmin.setup |config|   config.authentication_method = :authenticate_user!   config.authorization_adapter = activeadmin::cancanadapter   config.cancan_ability_class = "ability"   config.current_user_method = :current_user   config.logout_link_path = :destroy_user_session_path   config.allow_comments = false   config.batch_actions = true end 

restarting entire machine fixed this.

i had tried restarting both nginx , unicorn, had yet restart machine itself. whatever reason, did trick.

thanks thoughts / suggestions.